How can i replace missing mesh in UV editor?
Rather than fix the missing mesh, the best approach I found was to simply "reset" the unwrap, select all faces, then UV Unwrap again and use "Follow Active Quads". (hat-tip "moonboots"). I had to resize the mesh in the UV editor and align it onto the material. Other methods unwrap the selected mesh in ways that makes it unworkable to fill areas. When you initially unwrap a mesh, Blender (2.8) automagically lays out the mesh to match the boundaries of the material, but it appears that you can't repeat this process without manually aligning it.